The increasing multi-functionality of cellular phones is representative of the increasing volume of information processing in recent years, but while semiconductor processing performance has increased, capacitor performance has not kept pace in response thereto. The increase in information-processing performance could be referred to in other words as an increase in processing electrical current at higher frequencies. An increase in electrical capacitance is essential for handling increases in processing electrical current at high frequencies. For example, tantalum electrolytic capacitors are used in most cellular phones at present. These tantalum electrolytic capacitors have a larger electrical capacitance than aluminum electrolytic capacitors in low-frequency regions, but capacitance greatly decreases in high-frequency regions due to factors based on the sintered structure of these capacitors and falls short of the characteristics that are actually needed.
On the other hand, conventional solid aluminum electrolytic capacitors are incapable of obtaining greater electrical capacitance than tantalum electrolytic capacitors. Even though the high-frequency characteristics are better than in tantalum electrolytic capacitors, conventional solid aluminum electrolytic capacitors cannot handle large electrical currents in high-frequency regions. The aluminum material used in these aluminum electrolytic capacitors is usually molten aluminum that has a purity of 99.9% or more by mass and that is made into a slab by semi-continuous casting. A product having a thickness of 0.05 to 0.12 mm is completed after facing, homogenizing treatments, hot rolling, and, as necessary, process annealing and cold rolling. Depending on the maker, the surface area is then enlarged by alternating or direct current in a step referred to as “etching,” after which a dielectric film is formed on the surface in a chemical formation step, and an electrode for an electrolytic capacitor is fashioned. The electrical capacitance of a commercial aluminum electrolytic capacitor foil manufactured using such steps is approximately, e.g., 100 μF/cm2 for a low-voltage article formed at 20 V by a chemical process and 1.2 μF/cm2 for high-voltage article formed at 370 V by a chemical process. Increased capacitance for layered solid aluminum electrolytic capacitors is therefore attained by increasing the number of layers.
The etching in this case should involve dissolving the aluminum foil and thereby enlarging the surface area of the foil, allowing a higher electrical capacitance according to the depth of the etching, but when etching pits become connected during the progression of etching, high electrical capacitance will not be obtainable. Various studies involving, e.g., controlling components other than aluminum in these aluminum foils are accordingly being undertaken (see, e.g., Patent Documents 1, 2, 3, 4).    [Patent Document 1] JP-A 6-181146    [Patent Document 2] JP-A 2004-149835    [Patent Document 3] JP-B 3-61333    [Patent Document 4] Japanese Patent No. 3393607